Hi wondered if any of you tech wizards could help me with a small switching circuit. I want to piggy back off a flow sensor on a combo boiler to interface with a thermal store for the supply of hot water at the taps. The sensor voltage readings as follows earth 0V red Vcc 5V black with a yellow sensor wire reading 2.35 volts on activation. I need a high input resistance amplifier so as not to affect the existing circuitry ie minimal current drain which will switch state reliably with the 2.35 volt switching voltage which can be fed to a 12v relay to give 240v switching capacity. Low input voltage probably within noise region of TTL and probably below threshold for cmos or mosfet, with possible switching reliably issues hence some amplification may be required. Power supply available 12 volts DC. Look forward to any comments. Dave

I am not sure if I understood your question correctly, but as far as I could grasp. You have a sensor which operates on 5V and outputs 2.5V when triggered. With this sensor you have to turn on a 12V relay which will control an AC application and your operating system voltage is 12V.

So first thing first, power your sensor using a 5V voltage regulator like 7805 that will convert your 12V to regulated 5V. Then you need something that could turn on your relay when the sensor goes high. This something can be a transistor (MOSFET is really not required here) try searching for a transistor which has a VBE of less than 2.5V and can switch 12V. Also refer "relay driver circuit" to know how you should connect this transistor with a relay along with a diode for reverse current